918 



SPERM, OVA, AND PREGNANCY 



This thick and unusual basement membrane 

 adheres externally to a meshwork of tropho- 

 blastic giant cells. Maternal blood flows 

 through the interstices of this meshwork of 

 giant cells and presumably is a regional 

 source of some of the substances which gain 

 entrance to the vitelline circulation. A por- 

 tion of the parietal wall of the yolk sac is 

 firmly attached to the fetal surface of the 

 chorio-allantoic disc. (2) An inner, vascu- 

 lar, visceral wall (visceral splanchnopleure ) 

 is composed of a simple columnar endoder- 

 mal epithelium which rests on a mesenchy- 

 mal layer which carries the vitelline blood 

 vessels. A serosal basement membrane 

 (Wislocki and Padykula, 1953) separates 

 this mesenchymal layer from a narrow, 

 basophilic layer of mesothelium which lines 

 the exocoelom. As the allantoic vessels 

 penetrate the placental labyrinth, portions 

 of both visceral and parietal walls of the 

 yolk sac are invaginated into the labyrinth, 

 forming perivascular recesses which were 

 called "endodermal sinuses" by Duval 

 (1892). In the rat the parietal wall of the 

 yolk sac breaks down on the 15th day of 

 gestation, and this event makes the yolk sac 

 cavity confluent with the uterine cavity and 

 also puts the visceral endoderm into direct 

 contact with the uterine contents. 



On the basis of histophysiologic studies 

 on the absorption of dyes, Everett ( 1935 1 

 concluded that the yolk sac of the rat is a 

 significant organ of exchange and that it is 

 more permeable to dyes than the labyrinth. 

 Vital dyes, such as trypan blue, which are 

 relatively large molecules, find their way 

 rapidly into the yolk sac where they are 

 absorbed and stored by the visceral endo- 

 derm. These dyes reach the yolk sac, ap- 

 parently, either by way of the uterine mu- 



cosa or through that portion of Reichcrt's 

 membrane covering the fetal surface of the 

 allantoic placenta. Brambell and his co- 

 workers (1948, 1950, 1951, 1957) have es- 

 tablished experimentally in the rabbit and 

 rat that antibodies find their way from the 

 maternal circulation into the embryos, not 

 by passage through the thin and supposedly 

 more permeable layers of the chorio-allan- 

 toic placenta, but by way of the yolk sac 

 placenta, the latter mode of entry necessi- 

 tating transfer across several layers of cells 

 and tissues, including the structurally elab- 

 orate vitelline epithelium. Histologic evi- 

 dence in support of transport of antibodies 

 and serum proteins by the yolk sac placenta 

 comes from the localization of absorbed 

 serum proteins labeled by fluorescent dyes 

 (Mayersbach, 1958), and from autoradio- 

 graphic studies (Anderson, 1959). Both in- 

 vestigations substantiate the impermeabil- 

 ity of the labyrinthine trophoblasts to these 

 labeled proteins and demonstrate the accu- 

 mulation of proteins in the visceral endo- 

 derm. Brambell and Halliday (1956) and 

 Alayersbach (1958) have suggested from 

 different kinds of evidence that the endo- 

 dermal sinuses of Duval may also partici- 

 l^ate in antibody transport. Padykula 

 (1958 » demonstrated a rise in the succinic 

 dehydrogenase activity of the visceral com- 

 ponent of the endodermal sinuses shortly 

 before term in the rat. 



The absorptive visceral endodermal cells 

 are interesting from the points of view of 

 both cytology and placentation, since these 

 cells are capable of transporting certain 

 large molecules, such as antibodies and 

 serum proteins, and withholding and segre- 

 gating other colloidal substances, such as 

 trypan blue. In the latter respect, they func- 



Plate 15.x 



Fig. 15.43. Semischematic drawing of a human placenta delivered at full term. Two cotyle- 

 dons are illustrated, bounded above by the so-called chorionic or closing plate (c.p.) and on 

 the sides and below by septa placentae (s.) and the basal plate (b.p.). Placental branches of 

 the umbilical blood vessels are seen in the closing plate and in the anchoring villi (o.v.). 



Fig. 15.44. A section through a delivered placenta at full term, showing a darkly stained 

 placental septum extending up from the base of the placenta and forming the boundary 

 between two cotyledons. At the top of the figure, blood vessels in the closing plate are ap- 

 parent. Buffered formalin fixative. Azan stain. X 3. 



Fig. 15.45. A photograph of a placenta at full term showing a portion of a placental septum 

 at a higher magnification. Buffered formalin fixation. Periodic acid-Schiff stain. By this 

 method the septum is seen to consist of darkly stained ground substance composed principally 

 of fibrin in which there are numerous lacunae containing faintly stained individual tropho- 

 blast.* or colonies of them. X 90. 



